This invention relates generally to a logic-based image processing method for size dependent filtering and more particularly to logic-based image processing to compensate for marking process characteristics such as blooming, and size and orientation dependent artifacts in a xerographic engine.
The following related applications are hereby cross referenced and incorporated by reference for their teachings:
xe2x80x9cUSING MULTIPLE DIGITALLY-PRODUCED EXPOSURE LEVELS TO COMPENSATE FOR LASER ABSORPTION IN READ IMAGE-ON-IMAGE XEROGRAPHY,xe2x80x9d Crean et at., U.S. Pat. No. 6,111,593.
xe2x80x9cMETHOD AND APPARATUS FOR COMPENSATION OF BLOOMING ARTIFACTSxe2x80x9d Lin at al., U.S. Pat. No. 6,285,463.
xe2x80x9cAUTOMATIC ENHANCEMENT OF PRINT QUALITY BASED ON FEATURE SIZE,xe2x80x9d Eschbach at al., U.S. Pat. No. 6,275,304.
The present invention contemplates the use of logic-based, morphological operations to isolate image structures requiring size dependent modification, such as features that are susceptible to blooming when reproduced by an output device such as a color xerographic image-on-image, or any other type, of marking engine. The blooming condition, resulting from the need to overexpose the photoreceptor for latter-developed colors that are imaged through an existing colorant, does not lend itself to correction by simple adjustment of xerographic parameters or simple color correction.
Heretofore, a number of patents and publications have disclosed logic-based image processing, the relevant portions of which may be briefly summarized as follows:
Crawford, J. L., and C. D. Elzinga, xe2x80x9cImproved Output Quality by Modulating Recording Power,xe2x80x9d SPSE 41st Annual Conference, May 22-26, 1988, Arlington, Va. Discusses utilizing thickened strokes while performing smoothing, and the use of logical mask processing.
Loce, R. and E. Dougherty, Enhancement and Restoration of Digital Documents, SPIE Press, Bellingham Wash., 1997. Provides much tutorial information on logic-based image filtering and relevant morphological operationsxe2x80x94Section 1.5 teaches the basic relevant operations.
xe2x80x9cMethod and Apparatus for Digital Image Darkness Control Using Quantized Fractional Pixels,xe2x80x9d Inventors: R. Bracco, et al., Ser. No. 09/072,122 (May 5, 1997 provisional application, February 1998 actual filing), D/97210P,
Barski, L., and R. Gaborski, xe2x80x9cImage Character Enhancement using a Stroke Strengthening Kernel,xe2x80x9d U.S. Pat. No. 4,791,679, Dec. 13, 1988. Teaches how a character stroke is strengthened by processing video image data with a 16xc3x9716 kernel, and moving the kernel one pixel at a time through the image. For each pixel position, sections of the kernel, are selectively filled with black pixels in proportion to the number of black pixels in each section, in accordance with a set of predetermined rules.
Crawford, J., and J. Cunningham, xe2x80x9cBoldness Control in an Electrophotographic Machine,xe2x80x9d U.S. Pat. No. 5,128,698, Jul. 7, 1992. Control over the placement of an image edge location on the photoconductor of an electrophotographic machine as providing for a range of discharge levels for edge picture elements (PELS) which vary from greater than, to less than, that level used for fully discharged PELS. Such control is achieved independently of machine parameter control by altering edge PEL illumination intensity in accordance with data representing desired edge PEL intensity as the photoconductor sensitivity changes. A system for measuring and controlling the fully discharged PEL level establishes a measure of photoconductor sensitivity and is used for enabling the selection of current edge PEL intensity. Used control of marking process parameters, as opposed to modifying the digital image.
Mailloux, L., and T. Robson, xe2x80x9cDilation of Images without Resolution Conversion for Printer Characteristics,xe2x80x9d U.S. Pat. No. 5,483,351, Jan. 9, 1996. An image compensation system which provides dilation or erosion of image features using halfbitting or fullbitting in the rendition of bitmap images, especially on a write-white printer. A region of pixels of an image is isolated which includes two or more correctable pixel locations. A set of state determination rules, based on the formation of pixels in the isolated region, is used to determine a corrected binary pixel state for each of the correctable pixels. Corrections for one correctable pixel may be considered in the state determination rules for adjacent correctable pixels. A single enhanced output pixel is provided for each image input pixel, thereby preserving the original image resolution. Performing enhancements on multiple input pixels. Teaches employment of xe2x80x9chalfbitsxe2x80x9d to thicken strokes by a factional amount while maintaining printer resolution.
Murata, K., xe2x80x9cImage Processing Method and Apparatus,xe2x80x9d U.S. Pat. No. 5,450,208, Sep. 12, 1995. The image processing apparatus for smoothing edges in a reproduced image includes an image data generating circuit for generating image data including a specified pixel and a plurality of pixels surrounding the specified pixel; a sub-pixel data generating circuit for dividing the specified pixel included in the image data into N sub-pixels, for detecting the condition of the specified pixel and the condition of the plurality of pixels surrounding the specified pixel included in the image data by matching the image data with a plurality of predetermined patterns, and for generating sub-pixel data for determining the number and position of sub-pixels to be exposed of the N sub-pixels, based on the condition of the specified pixel and the condition of the plurality of pixels surrounding the specified pixel; and supplying circuit for supplying the sub-pixel data to exposure circuit which makes exposure. The sub-pixel data generating circuit generates sub-pixel data for exposing M sub-pixels of the N sub-pixels, when the specified pixel is detected to be an exposed pixel which requires no exposure correction, where M is smaller than N. The invention here relates to an image processing method and an image processing apparatus for smoothing jagged edges of characters, etc., and for stably reproducing thin lines and isolated dots, so as to achieve an image reproduction of high quality and to achieve an ideal tone characteristic by correcting the tone characteristic of digital halftone images.
Image-on-image (IOI) marking engines, where images are sequentially exposed and developed, typically produce a xe2x80x9cbloomingxe2x80x9d artifact in the later-imaged colors (e.g., magenta and cyan). Unfortunately, the blooming artifact does not easily lend itself to correction merely by adjusting the controls or setpoints of the xerographic engine. On the other hand, it has been discovered that it is possible to employ an image processing solution that will reduce the blooming artifact to an acceptable level. A morphological, or logic-based, image processing method may be employed to compensate for the loss of shadow detail associated with an observed blooming artifact.
In an image-on-image xerographic marking engine, the magenta and cyan separations are typically developed over the yellow separation. In IOI, the exposure level for magenta and cyan is therefore increased, compared to the yellow separation exposure level, to compensate for the transmission loss when the latter separation exposure occurs through the developed yellow image (yellow toner). However, in regions of the photoreceptor surface where there is no yellow toner, the exposure level of the latter separations will likely be too high-resulting in excessive line growth and loss of shadow detail called blooming. Blooming causes xe2x80x9cholesxe2x80x9d within a latent image to fill in with toner, which in turn results in the loss of shadow detail in halftoned images.
Although it may be possible to correct the exposure intensity on a pixel-by-pixel basis, the level of registration accuracy and hardware complexity necessary to enable such a correction is not readily achievable in commercial equipment. Similarly, other methods may be utilized to minimize the blooming artifacts. One such method, an object of the present invention, is the use of logic-based non-linear or (morphological) methods to adjust the image bitmap in a manner that corrects for the loss of shadow detail. More specifically, the present invention is directed to the enlargement of certain features (e.g., holes) within an image. Enlargement allows the area about the xe2x80x9cholesxe2x80x9d to be exposed and developed, or printed, without filling in the holes. Generally, the invention first isolates the regions of the image most likely to be affected by blooming using a logic-based xe2x80x9csiftingxe2x80x9d operation, then processes the regions to compensate for blooming, and merges or links the processed image with the original image to produce a digital image that will be blooming artifact reduced when printed with an image-on-image marking engine.
While blooming contributes to a particular size dependant artifact, other marking process attributes contribute to other size dependent artifacts. For instance, a marking process may print with narrow dark lines that are too thin, or wide dark lines that are too thin, or lines of a particular width in a given orientation are too thin or too thick. Some combination of size and orientation dependent artifacts may occur. A key aspect of the present invention is the generalized process of size and orientation dependent filtering for compensation of such artifacts, or for generation of a preferred rendition. This generalized operation may be thought of as a sieving operation, where image features of a particular size and orientation are sieved into classes, each class is modified, and the modified feature is input to the final processed image.
Another aspect of the invention is based on the recognition of the problem of blooming artifacts that appear in a display or other marking engine like an image-on-image xerographic printing system. More particularly, the invention utilizes image processing techniques to compensate for the blooming artifacts. The techniques employ logic-based, morphological filters to identify structures susceptible to blooming and other printing artifacts and then modify the structures to compensate for those artifacts before the digital image is rendered.
In yet another aspect of the invention, in addition to compensation for marking process characteristics, the present invention performs size and orientation filtering to produce an image that may produce a preferred rendition, possibly for some psychophysical reason or preference.
It will be well understood by those skilled in the art that the compensated image may be written not only to a printing device or marking engine, but may also be written to a display device or to a storage device for subsequent retrieval and use.
In accordance with yet another aspect of the present invention, there is provided a method for processing a digital image prior to printing the image, including the steps of isolating regions of the digital image most likely to be affected by blooming or other size-dependent marking artifacts, modifying the isolated regions to compensate for those artifacts; and merging the modified image features with unmodified image features in the digital image to produce an output digital image that is artifact free when printed using an marking engine, such as an image-on-image device.
In accordance to the present invention, there is provided a method for processing a digital image prior to printing the image on a given marking engine, including: isolating regions of the digital image most likely to be affected by an undesirable printing characteristic; producing a modified image by modifying the isolated regions to compensate for the undesirable printing characteristic; and merging the modified image with the digital image to produce an output digital image that is artifact free when printed using the given marking engine.
In the alternative, there is disclosed a method for processing a digital input image prior to printing the image on a given marking engine, including the steps of: isolating features that are not to be subject to a particular modification, modifying the remainder of the image with a morphological filter, and merging together the modified and unmodified features. For example, suppose it is necessary to thin most features of an image to compensate for blooming. Fine foreground feature regions of the digital input image would be adversely affected by such a compensation operation. In this case, a morphological filter is used to isolated the fine foreground filters, and a second morphological filter, such as an erosion is used to perform the compensation on the remaining image features. Then a subsequent, merging operation is performed to combine the unmodified fine foreground features with the compensated image features, thereby producing an output digital image that is artifact free when printed using the given marking engine.
These techniques can be implemented with a machine that prints color images from digital image data, including digital printers such as laser printers and facsimile machines. These techniques may also be utilized for displays or to images which are subsequently stored for later use. The technique described above is advantageous because it is efficient and can be accomplished using programmable image processing components. It is also flexible and can be adapted to compensate for any of a number of image structures that may result in visually perceptible printing artifacts.